CHOROID PLEXUS. 375 



tmn coating of white substance on its surface ; it has received its name 

 from giving origin to one root of the optic nerve. It is the inferior gan- 

 glion of the cerebrum. Part only of the thalamus is seen in the floor of 

 Sie lateral ventricle ; we must, therefore, defer its further description until 

 ,ve can examine it in its entire extent. 



The Choroid plexus (x v , s$e, resembling the chorion*) is a vascular 

 fringe extending obliquely across the floor of the lateral ventricle, and 

 sinking into the middle cornu. Anteriorly, it is small and tapering, and 

 communicates with the choroid plexus of the opposite ventricle, through 

 a large oval opening, the foramen ofMonro, or foramen commune ante- 

 rius. This foramen may be distinctly seen by pulling slightly on the 

 plexus, and pressing aside the septum lucidurn with the handle of the 

 knife. It is situated between the "under surface of the fornix, and the an- 

 terior extremities of the thalami optici, and forms a communication trans- 

 versely between the lateral ventricles, and perpendicularly with the third 

 ventricle. 



The choroid plexus presents upon its surface a number of minute vas- 

 cular processes, which are termed vilii. They are invested by a very 

 delicate epithelium, surmounted by cilia, which have been seen in active 

 movement in the embryo. In their interior the plexuses not unfrequently 

 contain particles of calcareous matter, and they are sometimes covered by 

 small clusters of serous cysts. 



The Corpus fimbriatum is a narrow white band, which is situated im- 

 mediately behind the choroid plexus, and extends with it into the de- 

 scending cornu of the lateral ventricle. It is, in fact, the lateral thin edge 

 of the fornix, and being attached to the hippocampus major in the de- 

 scending horn of the lateral ventricle, it is also termed, tenia hippocampi. 



The Fornix is a white layer of medullary substance, of which a portion 

 only is seen in this view of the ventricle. 



The Anterior cornu is triangular in its form, sweeping outwards, and 

 terminating by a pocnt in the anterior lobe of the brain, at a short distance 

 from its surface. 



The Posterior cornu or digital cavity curves inwards, as it extends back 

 into the posterior lobe of the brain, and likewise terminates near the sur- 

 face. An elevation corresponding with a deep sulcus between two convo- 

 lutions projects into the area of this cornu, and is called the hippocampus 

 minor. 



The Middle or descending cornu, in descending into the middle lobe of 

 the brain, forms a very considerable curve, and alters its direction several 

 times as it proceeds. Hence it is described as passing backwards and 

 outwards and downwards, and then turning forwards and inwards. This 

 complex expression of a very simple curve has given origin to a symbol 

 formed by the primary letters of these various terms ; and by means of 

 this the student recollects with ease the course of the cornu, BODFI. It is 

 the largest of the three cornua, and terminates close to the fissure of Syl- 

 vius, after having curved around the crus cerebri. 



The middle cornu should now be laid open, by inserting the little fingei 

 into its cavity, and making it serve as a director for the scalpel in cuttin-g 

 away the side of the hemisphere, so as to expose it completely. 



The Superior boundary of the middle cornu is formed by the under sur- 



* See the note appended tft the description of the choroid coat of the eye-ball 



